Velocity modulated electron discharge devices



Feb. 19, 1957 B. c. GARDNER VELOCITY MODULATED ELECTRON DISCHARGE DEVICES Filed March 10, 1952 Dua Y z,7sz,3s4 f VELOCITY MODULATED ELacrnbN DISCHARGE DEVICES Bernard C. Gardner, Los Altos; Caliigf'assgnor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application Maren 1o, 1952, stan No. 275,808 3 claims. (ci. sis-si) This invention relates to electron discharge devices of the velocity modulated type, and more particularly relates to a means for reducing hysteresis in reflex klystron oscillators.

ln velocity modulated oscillators of the reflex klystron type, an electron beam from a cathode is projected through a gap to which a tuned circuit, such a cavity resonator, is connected. An R. F. field exists across this gap so that the beam of electrons is velocity modulated on emerging from the gap. The electron beam next passes through a region between the gap and a reflecting electrode or repeller. In this reflector region the 'electrons are stopped by a strong retarding field and the direction of electron travel is reversed. The refiected electrons returning to the gap may be made` to deliver 'energy to the resonator owing to the interaction of the modulated beam current and the R; F. field across the gap.

in the normal klystron there are always some electrons that make more than 'one return transit. These multipletransit electrons give rise to hysteresis, thatV is, the phenomenon iny which the amplitude and yfrequency of oscillation depend upon the direction from which thev operating value of the klystron reliector yvelu-ige is approached. For

example, it has been' found that' the characteristic curves of power output versus repell'erelectrode potential for a given reflex klystron differ dependingr on whether the repeller or reflecting electrode potential is made progressively less negative with respect to the cathode.

To reduce this undesirable non-uniform operating characteristic and also to prevent mode asymmetry which is associated with hysteresis, a small elongated spike, such as a tungsten wire or metal rod, may be mounted in the center of the repeller. This spike causes a force to be exerted on the electrons radially outward from the beam axis. The resulting divergence of the beam, although not sufiicient to prevent electrons from returning through the R. F. gap, is great enough to prevent a reflection from the klystron cathode.

v conductive material, such as copper, which are `supported by thin discoidal portions 11 and 12 to which the glass The spike repeller, however, is useful only on short range tubes, that is, klystrons having relatively narrow frequency range of oscillation. rl`his limitation results vrom the fact that this structure has very poor depth field uniformity. When a spike .is used, the equi-potential lines of the fieldk between the grids and the vrepeller are not uniformly spaced andthe equi-potential surfaces adjacent the resonator are substantially fiat while those near the repeller electrodes are irregular in contour. As the repeller voltage is varied to accommodate a variation in frequency, the entire field is shifted. Because of the v non-uniformity of the field, a variation of frequency will 2,782,334 new ,Fe er1-2u lCe electrons, which are productive of hysteresis, to have aneffect on the oscillatonvthey must arrive at the gapsome- 5 what hunched and in a particular phase tothe oscillation across the gap. The eectof` the unevenly distributed or lumpy fieldvwill be to vary the depth of 'penetration of the electron bunch near the cathode, which will cause a spread in transit time of the electrons within a particular bunch. This will phase scramble the second transit electrons so that they will enter the gap from the cathode direction the second time as a uniform distribution, rather than in bunches, thereby preventing them from adding to or subtracting from the cavity oscillation.

In the drawing:

Fig. l is a longitudinal section view taken through the center of an electron discharge device according to the invention;

Fig. 2 is a plan view of a first type of cathode structure as used in the electron discharge device of Fig. l;

Fig. 3a is a fragmentary plan view of a second type of cathode structure as used in the electron discharge device of Fig. l; and

Fig. 3b is a fragmentary view in elevation of the cath- Vode structure shown in Fig. 3a.

Referring now to Fig. l of the drawing, T represents an electron discharge device of the velocityy modulated type, known more particularly as a reflex 'klystron An electron beam is produced by means of an electron gun which in the bottom portion of a Vsealed glass `envelope 7 mounted on tube base 8. v v

The electron beam passes through a gap 17 formed by a pair of spaced grid structures 9 and 1d made of highly envelope 7 is sealed; these discoidal portions are the means by which connection is made to an external tuned circuit, such as a toroidal cavity resonator. Since the latter forms no part of this inventiomit has not been Y illustrated. The first grids structure 9 includes, in addition yto discoidal portion 11, cylindrical tubular portion 13 having a narrow inwardly fianged portion 14 at the 'end opposite said discoidal portion. A mesh or screen 15 is supported between fianged portion 14 of grid structure 9 and retaining ring 16. A second grid structurel() is substantially similar to grid structure 9 vand comprises a discoidal portion 12, aanged portion 18, a mesh 19 and f retaining ring 20. v y

A repeller or reflecting electrode 21 is supported', for example, by aV conductor 22 sealed in the end of tube 1 opopsite base 8, as shown in-liig. 1. The repeller is spaced from grid structure 1t? and mounted coaxial with the cathode and grid structures. When a suitable voltage is applied to repeller electrode 21, the electrons in the electron beam which have passed through grids 9 land 10 are reflected baci` toward said grids. Some of these electrons will reach the cathode region and will be reflected therefrom. Because of the unevenly distributed field around the cathode, the second transit electrons'from the cathode direction arey phase scrambled so that the electrons enter the gap as a uniform distribution vrather than inbunches. i A l Fig. 2, which is an enlarged plan view of the cathode,

i f' 2,782,334 ,Y Y t.

illustrates one manner of irregular cathode coating. Electron emissive material 3 is supplied irregularly to the surface 2 of the cathode, as clearly shown in Fig. 2.

The irregular cathode coating may be produced in a large number of ways, 'such as scraping various areas from a uniformly coated cathode, spraying the cathode through a stencil, breaking the cathode down on exhaust in such manner as to cause coarse sintering, poisoning of various areas, or placing a metallic grid over the cathode. Any other means for providing avariable emission across the cathode is within the scope of the present invention.

A second type of irregular cathode is shown in plan and elevation in Figs. 3a and Fig. 3b, respectively. The cathode 2 is non-uniformly shaped and has alternate projections 25 and depressions 26 similar to the surface of a wafile iron.

With a non-uniformly coated cathode, the disturbance of the equi-potential lines is controlled by the cathode emission space charge adjacent the coated areas. The disturbance of the equi-potential lines by said space charge is not as effective as an irregular cathode geometry, as exemplied by the cathode of Fig. 3; an irregularly shaped cathode is therefore preferable to an irregularly coated cathode.

The principle according to the present invention is not to be limited to the type of klystron herein illustrated and described but may be used in any klystron in which ditliculties owing to hysteresis occur. Furthermore, the cathode configuration and cathode surface are not to be limited to those shown in Figs. 2 and 3. It should also be understood that the cathode may be constructed so that both the cathode geometry and the coating on the cathode are irregular.

What is claimed is:

l. An electron discharge device of the reflex klystron type comprising an electron gun including a cathode for emitting a single composite beam of primary electrons, a substantially planar rellecting electrode positioned lin the path of said electrons, a pair of spaced electron-permeable members defining a unitary gap traversed by said beam, said cathode having an electron emissive surface of irregular configuration and including a series of projecting areas and depressed areas for providing an electron beam density in the vicinity of said cathode which varies in random manner from point to point throughout the crosssectional area of the beam as presented to said gap whereby hysteresis in said device is substantially reduced.

2. An electron discharge `device of the reflex klystron type comprising an electron gun including a cathode for emiting a single composite beam of primary electrons, a substantially planar reecting electrode positioned in the path of said electrons, a pair of spaced electron permeable members defining a unitary gap traversed by said beam, said cathode having an electron emissive surface of irregular configuration including a series of projecting areas and depressed areas which have an irregular layer of thermionic emissive material thereon for providing an electron beam density in the vicinity of said cathode which varies in random manner from point to point throughout the cross-sectional area of the beam as presented to said gap whereby hysteresis in said device is substantially reduced.

3. An electron discharge device of the reflex klystron type comprising an electron gun including a cathode for emitting a single composite beam of primary electrons, a substantially planar reflecting electrode positioned in the path of said electrons, a pair of sapced electron permeable members defining a unitary gap traversed by said beam, said cathode having an electron emissive surface of irregular configuration and including a series of projecting areas and depressed areas which have a substantially uniform layer of emissive material thereon for providing an electron beam density in the vicinity of said cathode which varies in random manner from point to point throughout the cross-sectional area of the beam as presented to said gap whereby hysteresis in said device is substantially reduced.

References Cited in the file of this patent UNTTED STATES PATENTS 1,797,990 Lucian Mar. 24, 1931 2,180,710 Knoll et al. Nov. 2l, 1939 2,207,846 Wolff July 16, 1940 2,254,128 Van Den Bosch Aug. 26, 1941 2,292,437 Farnsworth Aug. 11, 1942 2,322,807 Iams June 29, 1943 2,459,792 Chevigny lan. 25, 1949 2,506,627 Barford May 9, 1950 2,581,408 Hamilton Ian. 8, 1952 

